System for transmission line termination by signal cancellation

ABSTRACT

A communication system having first and second states for use with a shared transmission line composed of at least two conductors and composed of first and second transmission line segments connected to each other at a single connection point. In the first state, a termination is coupled to the single connection point and is operative to at least attenuate a signal propagated between the first and second segments. In the second state, a driver is coupled to the connection point and is operative to conduct a signal over the first and second segments.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. application Ser. No. 12/724,952, filed onMar. 16, 2010, which is a continuation of U.S. application Ser. No.12/252,025, filed Oct. 15, 2008, now allowed, which is a continuation ofU.S. application Ser. No. 12/026,321, filed Feb. 5, 2008, now U.S. Pat.No. 7,453,284, issued on Nov. 18, 2008, which is a continuation of U.S.application Ser. No. 11/346,396, filed on Feb. 3, 2006, now U.S. Pat.No. 7,336,096, issued on Feb. 26, 2008, which is a division of U.S.application Ser. No. 11/100,453, filed on Apr. 7, 2005, now U.S. Pat.No. 7,068,066, issued on Jun. 27, 2006, which is a continuation of U.S.application Ser. No. 10/380,538, filed on Sep. 12, 2001, now U.S. Pat.No. 6,937,056, issued on Aug. 30, 2005, which is a national stage ofPCT/IL01/00863, filed on Sep. 17, 2000, the disclosures of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to the field of electrically-wiredcommunication, and, in particular, to communication lines employingtermination.

BACKGROUND OF THE INVENTION

The term “data unit” herein denotes any data processing device, such asa computer or a personal computer, including workstations or other dataterminal equipment (DTE) with an interface for connection to any wiredcommunication network, such as a Local Area Network (LAN).

Transmission lines over which digital signals are transmitted must beproperly terminated in order to prevent overshoot, undershoot andreflections. These effects, when caused by impedance mismatch, becomemore pronounced as the length of the conductor increases, and limit therate at which data can be transmitted over a transmission line. Thetransmission line can be a trace on an integrated circuit, a trace on aboard, or a wire in a cable. The impedance of both the source and loadshould be matched to the characteristic impedance of the transmissionline. Since the output impedance of a transmitter and the inputimpedance of a receiver generally differ from the characteristicimpedance of a transmission line interconnecting the transmitter and thereceiver in a point-to-point configuration, it is necessary to alter theexisting impedance differently at the source and load ends of thetransmission line.

Wire-based communication networks commonly employ terminations in orderto avoid reflections. An example of termination within a network isshown in FIG. 1. A shared wired network 10 is based on a two-wiretransmission line having wires 15 a and 15 b. In the followingdescription, reference will be made to “transmission line 15 a and 15b”, it being understood that the reference numerals actually refer tothe wires forming the transmission line. For example, network 10 may bean EIA/TIA-485 standard type, wherein transmission line 15 a and 15 bconsists of a single twisted pair, or an Ethernet IEEE802.3 standard10Base2 or 10Base5, wherein transmission line 15 a and 15 b is a coaxialcable. In general, the term ‘transmission line’ herein denotes anyelectrically-conductive media capable of carrying electrical current andvoltages, and transporting electromagnetic signals, including withoutlimitation wires, cables, and PCB traces. Differential line drivers 11 aand 11 b are used in order to transmit signals to the transmission line,while line-receivers 12 a and 12 b are used to receive signals carriedover transmission line 15 a and 15 b. Data unit 16 a is a “transmitonly” unit, which transmits data to the transmission line via linedriver 11 a, and data unit 16 b is a “receive only” unit that receivesdata from the transmission line via line receiver 12 a. Data unit 16 ccan both receive data from and transmit data to the transmission line 15a and 15 b via line diver 11 b and line receiver 12 b, forming atransceiver 14. Of course, additional units can be connected to sharedtransmission lines, each such units employing a line receiver, a linedriver, or both. In order to allow for proper operation of network 10,terminators 13 a and 13 b are commonly installed and connected to bothends of transmission line 15 a and 15 b. In order to function properly,terminators 13 a and 13 b should be equal in impedance to thecharacteristic impedance of transmission line 15 a and 15 b. Similarly,such terminations are employed in both ends of a point-to-pointconnection.

The need for termination is a major drawback in building a network.First, the transmission line ends must be identified and accessed, whichmay not be simple in the case of existing wiring. Additionally,terminator installation requires both labor and materials, and there isalso the issue of additional equipment required to configure a network.Furthermore, for proper operation, the termination type, topology andvalues are mainly based on the transmission line characteristics, whichmay be unknown and/or inconsistent, and may vary from cable to cable orfrom location to location.

An additional drawback of network 10 relates to being a multi-pointshared transmission line network. In a Time Domain Multiplexing (TDM)scheme, only a single driver can transmit over the transmission lineduring any time interval, rendering other units as receive-only duringthat time interval. This limits the total volume of data that can betransported over a specified period. In order to allow multiple datatransport over this shared transmission line, it is necessary to allowmultiple transmitters and receiver to use the transmission linesimultaneously.

One common method for such multiple transmissions over sharedtransmission line employs the Frequency Domain Multiplexing (FDM)scheme, wherein each transmitter uses a different dedicated portion ofthe transmission line's available spectrum. Such a solution, however,requires complex and expensive circuitry.

Another method for enabling multiple transmissions is shown in FIG. 2,and involves splitting the transmission line into distinct segments. Anetwork 20 is shown in part, wherein the transmission line is split intotwo distinct portions, one of which is identified as transmission linesegment 15 a and 15 b (as in FIG. 1), while the other portion isidentified as a transmission line segment 15 c and 15 d. Transmissionline segment 15 a and 15 b is used for full duplex communication usingline drivers 11 a 2 and 11 b 1, located at respective ends oftransmission line segment 15 a and 15 b. Similarly, line receivers 12 band 12 a 2 as well as terminators (not shown) are installed at therespective ends of transmission line segment 15 a and 15 b. Line driver11 a 2 and line receiver 12 a 2 are both part of a unit 21 a, which isconnected at one end of transmission line segment 15 a and 15 b.Similarly, transmission line segment 15 c and 15 d is coupled to linedrivers 11 c 1 and 11 b 2, as well as to line receivers 12 c 1 and 12 b2. Line driver 11 c 1 and line receiver 12 c 1 are both part of a unit21 c, connected at one end of transmission line segment 15 c and 15 d.Line drivers 11 b 2 and 11 b 1, as well as line receivers 12 b 1 and 12b 2 are all part of a unit 21 b, connected to transmission line segment15 a and 15 b, and to transmission line segment 15 c and 15 d. These twodistinct transmission line segments as well as their relateddrivers/receivers are coupled by a logic block 22, which is part of unit21 b. In certain prior art configurations, the logic block is eitheromitted or acts as transparent connection. In such case, unit 21 bserves as a repeater. In other configurations, logic block 22 processesthe data streams flowing through unit 21 b.

Network 20 offers two major advantages over network 10 as shown inFIG. 1. First, each transmission line segment of network 20 isindependent, allowing two communication links to operate simultaneously.Hence, line driver 11 a 2 of unit 21 a can transmit data overtransmission line segment 15 a and 15 b, to be received by line receiver12 b 1 of unit 21 b. Simultaneously, and without any interference, linedriver 11 c 1 of unit 21 c can transmit data over transmission linesegment 15 c and 15 d to be received by line receiver 12 b 2 of unit 21b.

Yet another advantage of network 20 is that of having point-to-pointcommunication segments. As is well known in the art, point-to-pointtopology is a highly favored configuration in wired communication,enabling robust, high bandwidth communications with low-cost, simplecircuitry.

Principles of the above description are demonstrated by the evolution ofthe Ethernet Local Area Network (LAN) as specified in the IEEE802.3standard, wherein shared transmission line systems based on coaxialcable 10Base2 and 10Base5 were upgraded towards 10BaseT and 10BaseTXbased networks, both built around point-to-point segments.

However, network 20 also exhibits a major disadvantage in comparison tonetwork 10. As shown in FIG. 1, network 10 uses a continuousuninterrupted transmission line. In contrast, the wiring of network 20must be cut at several points throughout the network, wherein units 21are simply connected. In the case of existing transmission lines (suchas in-wall telephone wiring), cutting into the network may be complex,expensive, and labor-intensive.

There is thus a widely recognized need for, and it would be highlyadvantageous to have, a means for implementing a generic terminationthat is not transmission line-dependent, and which therefore would notneed to be changed when the transmission line characteristics change.There is also a widely recognized need for a means for simultaneousmultiple use of a single wiring infrastructure, and for employing apoint-to-point connection scheme, without modifying such existingwiring. These goals are addressed by the present invention.

SUMMARY OF THE INVENTION

The invention relates to a system and method for signal termination,based on a two-port unit, denoted herein as a Signal Canceling Unit(SCU). The SCU senses the signal present over its terminal, and operatesto absorb and cancel this signal. When connected at an end of atransmission line, such as a wire transmission line used forcommunication, the SCU functions as a terminator by absorbing the signalenergy. When connected in the middle of such wiring transmission line,the SCU terminates any signal sensed over its terminals, and thus can beused for noise isolation, or to emulate a network end in the connectedpoints. In this functional mode, the SCU effectively splits the wires,allowing for different independent networks operation at each side ofthe SCU connection, without interfering or interacting with each other,even though the continuity of the wiring is not affected.

In another embodiment, the SCU is upgraded to include line receiverfunctionality, denoted herein as a Signal Canceling and Receiving Unit(SCRU). In addition to having full SCU functionality, the SCRU alsooperates as a line receiver, and hence can be used as an active receiverin the network, in addition to serving in termination and signalcanceling roles.

In yet another embodiment, the SCRU is upgraded to include line driverfunctionality, denoted herein as a Signal Canceling, Receiving, andTransmitting Unit (SCRTU). In addition having full SCRU functionality,the SCRTU also performs as a line driver, and hence can be used as anactive transmitter in the network, in addition to serving intermination, signal canceling, and receiving roles. Multiple SCRTU'sconnected to wired transmission lines can communicate for constructionof a full network. In such a network, every pair of adjacent-connectedSCRTUs can communicate in a point-to-point fashion, in a terminated andindependent transmission line segment.

Therefore, according to a broad aspect of the present invention there isprovided a device for actively terminating and isolating a continuouslyconducting transmission line, said device comprising:

a sensor operative to sensing a first signal on the transmission line;

a first driver operative to placing a second signal on the transmissionline for canceling the first signal; and

a processing unit operative to receiving input from said sensor andproviding input to said first driver.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, a preferred embodiment will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIG. 1 shows a common prior art shared wired Local Area Networkconfiguration;

FIG. 2 shows a prior art repeater based communication network;

FIG. 3 shows a Signal Canceling Unit (SCU) functional block diagramaccording to a first embodiment of the present invention;

FIG. 4 shows a shared wiring based network, wherein an SCU is used as anend terminator according to the present invention;

FIG. 5 shows a shared wiring based network, wherein an SCU is used as aparallel connected terminator according to the present invention;

FIG. 6 shows a shared wiring based network, wherein an SCU is used fornoise isolating according to the present invention;

FIG. 7 shows a shared wiring based network, wherein an SCU is used forbridge-tap isolating according to the present invention;

FIG. 8 shows a shared wiring based network, wherein an SCU is used forallowing multiple independent communication segments over continuouswiring according to the present invention;

FIG. 9 shows a Signal Canceling and Receiving Unit (SCRU) functionalblock diagram according to a second embodiment of the present invention;

FIG. 10 shows a shared wiring based network, wherein an SCRU is used forallowing multiple independent communication segments over continuouswiring according to the present invention;

FIG. 11 shows a Signal Canceling, Receiving and Transmitting Unit(SCRTU) functional block diagram according to a third embodiment of thepresent invention;

FIG. 12 shows an alternative Signal Canceling, Receiving andTransmitting Unit (SCRTU) functional block diagram according to a fourthembodiment of the present invention; and

FIG. 13 shows a shared wiring based network, wherein multiple SCRTU'sare used for allowing multiple independent communication segments overcontinuous wiring according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The principles and operation of a network according to the presentinvention may be understood with reference to the drawings and theaccompanying description. The drawings and descriptions are conceptualonly. In actual practice, a single component can implement one or morefunctions; alternatively, each function can be implemented by aplurality of components and circuits. In the drawings and descriptions,identical reference numerals indicate those components that are commonto different embodiments or configurations.

FIG. 3 illustrates a Signal Canceling Unit (SCU) 30, which includes twoexternal terminal connections, a terminal 34 a (A) and a terminal 34 b(B). Coupled to these terminals is a sensor 31, which measures thedifferential voltage (constituting a “first signal”) between terminal 34a and terminal 34 b. The value measured by sensor 31 is input into aprocessing unit 33, which in turn provides input to a differentialdriver 32 (constituting a “first driver”), whose outputs are coupled tothe terminal 34 a and terminal 34 b. Driver 32 can sink or source enoughcurrent (constituting a “second signal”) to cancel the first signal atthe terminals. Processing unit 33 along with sensor 31 and driver 32forms a closed negative feedback loop, which attenuates and cancels anysignal sensed over terminal 34 a and terminal 34 b.

FIG. 4 illustrates a network 40, with SCU 30 used as a terminator.Network 40 is based on network 10 (FIG. 1), but modified to use SCU 30as a terminator in place of terminator 13 b. Signals transmitted totransmission line 15 a and 15 b (by line driver 11 a, for example)propagate along the transmission line. Upon reaching the end of thetransmission line, where terminals 51 a and 51 b of SCU 30 areconnected, SCU 30 senses and acts to cancel the signals. As a result,the signal energy is absorbed by SCU 30, and neither reflection nor anyother mismatch occurs. Hence, SCU 30 acts as a termination device.However, since the structure of SCU 30 is generic and is not tailored tothe specific transmission line (e.g., characteristic impedance), thissame SCU can be used for many types of transmission line, such astwisted pair wiring, coaxial cables, etc., obviating the need to match aspecific termination to a specific transmission line. This, of course,provides simple installation and easy logistics, due to the employmentof common components for various different applications.

A further advantage of using an SCU as a terminator stems from the factthat the SCU performs the termination function even when not connectedat the ends of the transmission line, but at any point throughout thetransmission line run, as illustrated in FIG. 5 for a network 50, whichis based on transmission line 15 a, 15 b, 15 c, and 15 d. As withnetwork 10 (FIG. 1), terminator 13 is located at one end (left side ofthe figure), and line driver 11 a and line receivers 12 a and 12 b arecoupled to the transmission line. Data units 16 a, 16 b, and 16 d arecoupled to line units 11 a, 12 a, and 12 b, respectively. If SCU 30 werenot present in network 50, network 10 of FIG. 1 would be obtained,wherein data unit 16 a can transmit data to the entire transmission linevia line driver 11 a. The transmitting signals would then propagate inthe transmission line and would be received by data units 16 b and 16 dvia line receivers 12 a and 12 b, respectively. In this case, however,where SCU 30 is connected to the transmission line at connection points51 a and 51 b, the network 50 is modified such that signals transmittedto line driver 11 a, are propagated in the transmission line in twodirections. Part of the signal energy is propagated towards terminator13 (towards the left side of the figure), where they are absorbed. Theother part of the signal energy propagates towards points 51 c and 51 d,representing the other end of the wiring. When the signal reaches points51 a and 51 b (connected to the terminals of SCU 30), SCU 30 operates toattenuate, cancel, and absorb the signal energy. Thus, little or nosignal will propagate from the points 51 a and 51 b towards the endpoints 51 c and 51 d. In such case, while line receiver 12 a willreceive the transmitted signals, line receiver 12 b will not sense anysuch signals, which are attenuated by SCU 30. Thus, SCU 30 functions asa terminator for the network segment 15 a and 15 b, extending fromterminator 13 to points 51 a and 51 b, helping to avoid reflections inthis part of the transmission line. As a result, SCU 30 modifies thefunctionality of the continuous transmission line to be virtuallyseparated into two distinct segments, one using the transmission linefrom terminator 13 to points 51 a and 51 b, while the other uses thetransmission line from points 51 a and 51 b to the end-points 51 c and51 d. The two network segments are isolated in the sense that signals inone segment cannot pass to the other, even though electrical continuityof the transmission line is fully retained.

One application of such virtual networks separation is for noiseisolation, as illustrated in FIG. 6 with a network 60. Network 60 issimilar to network 50 (FIG. 5), except that a noise source 61 appears inplace of data unit 16 d and line receiver 12 b. The noise generated bynoise source 61 propagates (in the left direction) towards SCU 30. Uponreaching SCU terminals 51 a and 51 b, SCU 30 operates to attenuate thenoise signal, and prevents the noise from reaching transmission line 15a and 15 b and thereby degrading communication over that networksegment. While noise source 61 is described and illustrated as adistinct unit connected at a single point to transmission line 15 c and15 d, the same noise cancellation function is performed where noise isgenerated by inductive means from external sources. For example,transmission line 15 c and 15 d may extend over an area near sources ofelectromagnetic interference. The SCU can thus help in isolating theinduced noise from a specific portion of the conductive transmissionline.

Bridge-taps are known to cause impedance mismatch and reflections intransmission lines and other wired communication environments. FIG. 7illustrates a network 70, which is similar to network 60 (FIG. 6), butwith added transmission line 15 e and 15 f, connected to terminals 51 aand 51 b respectively, forming a bridge tap at terminals 51 a and 51 b.Without SCU 30, the bridge tap at these points would create an impedancemismatch and cause signal reflections in the communications overtransmission line 15 a, 15 b, 15 c, 15 d, 15 e, and 15 f. The presenceof SCU 30 at the bridge-tap junction, however, cancels and absorbs thesignals at terminals 51 a and 51 b, and eliminates such reflections. Indoing so, three isolated communication segments are formed, one segmentconsisting of transmission line 15 a and 15 b, a second segmentconsisting of transmission line 15 c and 15 d, and a third segmentconsisting of transmission line 15 e and 15 f.

The capability of an SCU to isolate electrically connected transmissionline enables the formation of multiple distinct communication networksover continuous electrical conducting transmission line, as shown inFIG. 8. A network 80 is based on transmission line 15 a, 15 b, 15 c, and15 d. SCU 30 connects to the transmission line at terminals 51 a and 51b, and isolates the transmission line into two communication segments.One segment is based on transmission line 15 a and 15 b, and extendsfrom terminals 51 a and 51 b towards the left in FIG. 8. The othersegment is based on transmission line 15 c and 15 d, and extends towardthe right. Data unit 16 a transmits across transmission line 15 a and 15b via line driver 11 a, and provides the signal received by data unit 16b via line receiver 12 a. Similarly, data unit 16 e transmits acrosstransmission line 15 c and 15 d via line driver 11 b, with the signalreceived by data unit 16 d via line receiver 12 b. Being isolated by SCU30, both transmissions can occur simultaneously, without interferingwith each other. Additional line drivers, line receivers andtransceivers can be added to each communication segment. Similarly,adding additional SCU's can split electrically-connected transmissionline into more segments, wherein an isolated segment is formed betweenadjacent SCU pairs, or between the SCU and the ends or terminators oftransmission lines.

The function of the SCU has been so far been described only as aterminator, but an SCU can also be modified to perform a line receivingfunction, as shown in FIG. 9, which illustrates a Signal Canceling andReceiving Unit (SCRU) 90. SCRU 90 is based on the structure of SCU 30,(FIG. 3), but the processing unit 33 is modified to a processing unit91, which provides additional output via a terminal 34 c (C). The outputon terminal 34 c uses sensing function 31, and together with part ofprocessing unit 91 serves as a line receiver, similar to line receiver12 a or 12 b. Thus, SCRU 90 simultaneously performs two functions:signal cancellation as does SCU 30, and line receiver functionality, asdo line receivers 12 a and 12 b, thus allowing the sensed signal or anyfunction thereof to be output on the terminal 34 c and placed on thetransmission line.

An example of an application using SCRU 90 is shown in FIG. 10, for anetwork 100. Network 100 is based on network 80 (FIG. 8), but SCU 30 isreplaced by SCRU 90, whose terminal C is connected to a data unit 16 fvia a connection 102. SCRU 90 further is connected to transmission line15 a, 15 b, 15 c, and 15 d at junctions 101 a and 101 b. In a mannersimilar to that of network 80 (FIG. 8), this configuration allows twoisolated communication segments to use the transmission linesimultaneously without interfering with each other. One segmenttransports data over transmission line 15 a and 15 b, while the othersegment transports data over transmission line 15 c and 15 d. Inaddition, by utilizing the line-receiving functionality of SCRU 90, dataunit 16 f can receive signals from both networks.

In yet another embodiment of the invention, a line-driving capability isalso integrated into the SCRU. FIG. 11 illustrates an SCRTU (SignalCanceling, Receive and Transmit unit) 110. SCRTU 110 includes allcomponents of SCRU 90, but also includes a line driver 111 (constitutinga “second driver”), which is fed from an additional SCRTU terminal 34 d(D) and feeds a third signal to the transmission line. SCRTU 110 has twostates of operation, denoted as “receive” and “transmit”. In “receive”state, the functionality of SCRU 90 is fully retained, and SCRTU 110performs signal cancellation and reception. In “transmit” state, lineterminals 34 a (A) and 34 b (B) are connected to line driver 111 outputterminals as shown, so that SCRTU 110 can transmit data received atterminal 34 d to terminals 34 a and 34 b. Shifting between the states isperformed by two SPDT (single pole double throw) switches 112 and 113.Switches 113 and 112 are connected to terminals 34 a and 34 b,respectively. In the ‘receive’ state, both switches 112 and 113 are instate ‘1’, thus connecting terminal 34 a and terminal 34 b terminals tosensor 31 and driver 32, and thereby performing the function of SCRU 90.In the ‘transmit’ state, both switches 112 and 113 are in state ‘2’,thus connecting terminal 34 a and terminal 34 b to the outputs of linedriver 111, and thereby performing as a line driver. Switches 112 and113 are controlled by a logic unit 114, which changes switches 113 and112 as required to select the desired state.

FIG. 12 illustrates an alternative implementation of an SCRTU 120. Inthis alternative configuration, driver 32 is also used as the linedriver. An SPST switch 121 is used to route the input into driver 32. Instate ‘1’, driver 32 is connected to the output of processing 91, andthereby performing the function of SCRU 90. In state ‘2’, driver 32 iscoupled to terminal 34 d, and thereby functions as a line driver. Alogic block (not shown in FIG. 12) is used to control switch 121,shifting it from state to state as required.

FIG. 13 illustrates a network 130 using such SCRTU's. Network 130 usesnetwork transmission line 15 a, 15 b, 15 c, 15 d, 15 e, 15 f, 15 g, and15 h, and has a bridge-tap at points 51 a and 51 b. Data units 16 f, 16g, 16 h, 16 i, and 16 j are coupled to the transmission line via SCRTU's110 a, 110 b, 110 c, 110 d, and 110 e, respectively. As explained above,although the wiring is electrically continuous, the communicationsegments formed are of point-to-point type between any SCRTU pair. SCRTU110 a communicates in a point-to-point topology with SCRTU 11 b, overtransmission line segment 15 a and 15 b. Similarly, SCRTU's 110 b and110 e communicate over transmission line segment 15 e and 15 f, SCRTU's110 b and 110 c communicate over transmission line segment 15 c and 15d, and SCRTU's 110 c and 110 d communicate over transmission linesegment 15 g and 15 h. In addition to the benefit of point-to-point, thenetwork also allows for multiple independent communication segments tooperate independently, as long as there are not any two SCRTU'stransmitting to the same segment. For example, SCRTU 110 a can transmitto SCRTU 110 b over transmission line segment 15 a and 15 b, while SCRTU110 d can simultaneously transmit to SCRTU 110 c over transmission linesegment 15 g and 15 h.

Network 130 demonstrates the SCRTU based network capability ofpoint-to-point communications and multiple transmissions over continuouswiring. These capabilities can be useful for existing wiring havingunknown topology, and having ‘bus’ type connection points. For example,in-wall existing telephone wiring, in-wall existing power lines or CATVcabling which are not used for their original purpose. Continuity iscommon to all of these types of wiring, where outlets are provided forconnecting to the wiring. Hence, coupling SCRTU's to each outlet allowsfor reliable high bandwidth communication between data units connectedto the SCRTU's.

While the invention has been described with respect to a digitalcommunication application, it will be appreciated that the invention isequally applicable to analog communication as well, such as video, audioor any other type of communication. In such configurations, data units16 are replaced by suitable analog units, and the SCU's, SCRU's, andSCRTU's are modified accordingly to support such communication.

While the invention has been described with respect to a limited numberof embodiments, it will be appreciated that many variations,modifications and other applications of the invention may be made.

What is claimed is:
 1. A device for use with an existing power cable inwalls of a building and connected to a power outlet having a powerconnector, the power cable being connected in a bus topology forcarrying a digital data signal, said device comprising: a connector forconnecting to the power outlet for coupling to the digital data signalcarried over the power cable; and a termination circuit selectivelycouplable to said connector and constructed for terminating the digitaldata signal propagated over the power cable when said device isconnected to said power connector, wherein said device is switchablebetween a first state in which said termination circuit is coupled tosaid connector and a second state in which said termination circuit isnot coupled to said connector.
 2. The device according to claim 1,further comprising a sensor having a sensor input coupled to saidconnector, and a sensor output, said sensor being operative for sensingthe digital data signal at said connector and for supplying to saidsensor output a signal that is a function of the data signal that issensed.
 3. The device according to claim 1, wherein said terminationcircuit further comprises: a driver having a driver input and a driveroutput, said driver output being couplable to said connector for placinga signal on the power cable; and a processing unit coupled between saidsensor output and said driver input, and operative to supply said driverinput with a signal that is effective to at least attenuate the digitaldata signal on the power cable at said connector.
 4. The deviceaccording to claim 3, further comprising a receiver port coupled to saidprocessing unit, and wherein said processing unit is operative in saidfirst state to generate a signal that is a representation of theattenuated data signal and to supply the generated signal to saidreceiver port.
 5. The device according to claim 3, further comprising adata unit port coupled to said processing unit and operative forcoupling a data unit coupled to the data unit port to the power cable.6. A communication system switchable between first and second states foruse with a power cable in walls of a building in a bus topology andconnected for carrying digital data signal, said system comprising: apower outlet comprising a power connector; first and second power cablesegments forming portions of the power cable and connected to each otherat said power connector of said power outlet to form a continuoustransmission line; and a termination device selectively couplable tosaid power connector and, when coupled to said power connector,operative to at least attenuate the digital data signal propagatedbetween said first and second power cable segments, wherein in the firststate said termination device is coupled to the power connector, and inthe second state said termination device is decoupled from the powerconnector.
 7. The communication system according to claim 6, furthercomprising a sensor having sensor inputs coupled to said powerconnector, and a sensor output, said sensor being operative for sensinga signal at said power connector and for supplying to said sensor outputa signal that is a representation of the signal that is sensed.
 8. Thecommunication system according to claim 7, wherein said terminationdevice comprises: a driver selectively couplable to said telephoneconnector and, when coupled to said power connector, operative toconduct a signal over said first and second power cable segments.
 9. Thecommunication system according to claim 6, wherein said terminationdevice comprises a data unit port coupled to said power connector andcouplable to a data unit, said data unit port being operative to couplethe data unit to said first and second power cable segments.
 10. Thecommunication system according to claim 6, further comprising at least athird power cable segment connected to said power connector.
 11. Thecommunication system according to claim 10, wherein at least one of saidpower cable segments is open-ended at an end remote from said powerconnector.